Farida Benmouna

On the phase behavior of blends of polymers and nematic liquid crystals

The phase behavior of mixtures of polymers and nematic liquid crystals (LC) is investigated. Two types of systems are examined. The first one deals with blends in which the polymer is made of linear chains. In this case, a systematic study of the effects of various parameters on the phase diagrams is performed. In particular, it is shown how increasing the polymer size and/or the LC molecule size increases the miscibility gap of the mixture. It also reduces the region where a single nematic phase is observed in the presence of a tiny amount of polymer. Likewise, the relative effects of the isotropic and the nematic interaction parameters on the phase diagrams are examined. The second part of this investigation deals with blends involving crosslinked polymers. Here, substantial differences are observed as compared to the case where the polymer components are made of linear chains. These differences are illustrated by showing the phase diagrams in similar conditions for both blends. Unlike the case of a linear polymer matrix, it is observed that the single nematic phase and the nematic-isotropic spinodal branches are absent from the phase diagram of crosslinked polymers. This results into significant distortions of the phase diagram. In order to highlight all these effects, examples representing hypothetical blends are considered. These examples are chosen for illustration of the results in which the choice of numerical parameters is made consistently with the existing values in the literature which makes comparison with published data possible.

Equilibrium phase diagram of polystyrene and 8CB

The experimental equilibrium phase diagram of a mixture of linear polystyrene of molecular weight Mw = 44,000 g/mol and 4-cyano-4′-n-octyl-biphenyl (8CB) is established. The three transitions smectic A-nematic, nematic-isotropic, and isotropic-isotropic are observed. The first two are observed both by optical microscopy and differential scanning calorimetry (DSC) while the isotropic-isotropic transition could be seen only via optical microscopy. Two series of samples with the same compositions were independently prepared and yielded consistent results both by microscopy and DSC. Measurements of sample compositions with less than 50 weight % of 8CB were influenced by the vicinity of the glass transition temperature (Tg) of the polymer in the mixture. This quantity is also determined by DSC as a function of composition. A single Tg is observed, which decreases with composition of the LC. Other thermodynamic quantities such as the enthalpy variations of LC in the nematic-isotropic transition and the fraction of LC contained in the droplets are also considered.

Phase equilibrium of poly(n-butyl acrylate) and E7

The experimental equilibrium phase diagram of mixtures of linear poly(n-butyl acrylate) of molecular mass Mw = 112000 g mol-1 and the low molecular mass LC mixture E7 has been established using polarized optical microscopy and light scattering techniques. The diagram is found to be reminiscent of an upper critical solution temperature system. Two independent series of samples with the same composition were studied, yielding consistent results. A region of nematic and isotropic coexisting phases and a region of a single isotropic phase were identified in the composition-temperature phase diagram. The results were analysed within a theoretical model combining the Flory-Huggins lattice theory for isotropic mixing and the Maier-Saupe theory for nematic ordering. Interestingly, no region of isotropic coexisting phases was observed in our experiments. This is probably due to the fact that the nematic interaction overwhelms the isotropic interaction in the region where (I + I) coexisting phases could appear. A preferential solubility of certain constituents of the LC mixture in the polymer could possibly be a reason for this behaviour.

Phase diagrams of phenyl benzoate side group liquid crystal polymers and similar low molecular mass liquid crystals

The miscibility of a phenyl benzoate side group liquid crystal polymer (LCP) with a similar low molecular mass liquid crystal (LC) has been studied as a function of temperature and molecular mass by calorimetry and optical microscopy. The nematic to isotropic transition temperatures of the pure polymer and the LC are 112 and 48.8°C, respectively. There is a broad miscibility gap for polymer weight fractions below 50% and temperatures below 72°C. A relatively small increase in miscibility is observed when the polymer molecular mass decreases. In the systems investigated here, the experimental temperature/weight fraction phase diagram is well reproduced by a mean field model combining the Flory-Huggins model for isotropic mixing and the Maier-Saupe theory for nematic ordering. Consequences of the miscibility gap with regard to the behaviour of LCP brushes swollen in a similar low molecular mass nematic liquid crystal are discussed.

Hydrodynamics of particle–wall interaction in colloidal probe experiments: comparison of vertical and lateral motion

The hydrodynamic interaction between a colloidal particle attached to the tip of an atomic force microscope (AFM) and a wall has been investigated as a function of the angle of inclination of the cantilever with respect to the surface. For high inclination of the cantilever, the motion mostly occurs in the tangential direction. A frequency-dependent drag coefficient is extracted from the cantilevers Brownian motion. In agreement with theoretical predictions, the wall-induced drag for tangential motion is much weaker than that for vertical motion. Mechanical properties of the surface are more easily probed with tangential motion because the hydrodynamic effects do not mask the surfaces intrinsic properties as much as for vertical motion.

Equilibrium phase behavior of polymer and liquid crystal blends

A theoretical framework describing the equilibrium phase behavior of polymers and liquid crystals is presented. Linear and crosslinked polymers are considered, and complexities found in the phase properties of systems involving crosslinked networks are higlighted. Effects of the rubber clasticity parameters in the elastic free energy are found to induce substantial distortions in the phase diagram. The Flory-Huggins interaction parameter which governs the miscibility of the mixture in the istropic state is assumed to be independent of the polymer architecture and modeled either by using a function of temperature only or temperature and composition. The thermodynamic description of the ordered domains is made according to the Maier-Saupe theory for nematic order and its extension to include other ordering properties. In particular, the smectic-A order is described according to the generalization of the Maier-Saupe theory proposed by McMillan. In the presence of nematogens, the coupling leads to quite different phase properties. In the strong coupling limit, a wide single nematic phase is found. In the weak coupling, the miscibility gap is much wider. These mixtures are described with mean-field theories of nematogen first developed by Brochard et al. and later extended by Kyu et al. This theoretical formalism has been applied successfully to analyze data obtained on several systems including linear and crosslinked polymer networks, smetic and nematic low molecular weight liquid crystals (LMWLC), and nematogen mixtures.

Polymerization-induced Phase Separation: Phase Behavior Developments and Hydrodynamic Interaction

The process of polymer synthesis based on polymerization-induced phase separation (PIPS) is revisited from the theoretical point of view. Cahn–Hilliard–Cook theories for spinodal decomposition are adapted to describe the kinetics of phase separation and deduce the time-resolved scattering function, while the double reaction model is used to describe the kinetics of polymerization. Coupling of these two kinetics is provided by the Carothers equation relating the fraction of reacted monomers to the degree of polymerization at time t, denoted N(t). It is argued that the approach to criticality is governed by a critical parameter, χc, that is different from the usual parameter for spinodal decomposition, χs, deduced from the second derivative of the free energy. While the latter parameter depends on the reciprocal degree of polymerization N−1(t), the former one depends on its time integral. This leads to significant consequences on the phase behavior developments during the PIPS process. Hydrodynamic interactions are found to speed up the emergence of instability modes. Although the qualitative trends remain similar to those of the Rouse dynamics, important quantitative changes are found due to the long-range viscous flow effects.

Thermophysical behaviour of monofunctional acrylate and liquid crystal systems

Abstract The phase behavior of monofunctional acrylate and low molecular weight nematic liquid crystals (LC) is considered. Systems involving the monomeric 2-ethylhexylacrylate (2-EHA), the eutectic LC mixture known as E7 and the 4-cyano-4′-n-pentyl-biphenyl (5CB) are investigated. A similar investigation is performed on mixtures involving a polymer poly-2-EHA with molecular weight Mw=48,000 g/mol and both LCs. The experimental phase diagrams are established using polarized optical microscopy and analyzed using a theoretical formalism which combines the Flory–Huggins theory of isotropic mixing and the Maier–Saupe theory of nematic order. The results lead to characterization of the miscibility of E7 and 5CB with monomeric and analogous polymeric 2-EHA systems.

Phase diagrams of poly(dimethylsiloxane)/E7 mixtures

Abstract The phase behavior of blends of poly(dimethylsiloxane) and the eutectic mixture of liquid crystals E7 is investigated. Experimental phase diagrams are established by polarized optical microscopy for two systems with widely different polymer molecular weights. Surprisingly, we find a very slight loss of miscibility of the mixture as a result of an increase of the polymer molecular weight by almost an order of magnitude. This unexpected phase behavior is in contrast with the results reported so far on other systems.

Phase properties of hexanedioldiacrylate/E7 blends

Abstract Equilibrium phase diagrams of uncured and UV-cured difunctional hexanedioldiacrylate and the eutectic mixture of low molecular weight liquid crystals E7 are established by polarized optical microscopy and differential scanning calorimetry. Data are analyzed using the Maier-Saupe model of nematic order. In the case of the uncured system the Flory-Huggins free energy of isotropic mixing is applied while for the cured system, the Flory-Rehner elastic free energy model is used. A good correlation between experimental and theoretical phase diagrams is found in both systems.